Technical Field
The present invention relates to a semiconductor device (insulated gate field effect transistor) and a method for manufacturing the same.
Background Art
For example, a metal-oxide-semiconductor field effect transistor (MOSFET: insulated gate field effect transistor) or an insulated gate bipolar transistor (IGBT) has been known as a semiconductor element used in a power semiconductor device. FIG. 5 is a circuit diagram illustrating a general inverter. FIG. 6A is a cross-sectional view illustrating a main portion of a general IGBT and FIG. 6B is a cross-sectional view illustrating a main portion of a MOSFET. An IGBT 101 has come into widespread use as a high-breakdown-voltage switching element which is used for an inverter circuit 1000 illustrated in FIG. 5. The IGBT 101 has excellent features, such as the high breakdown voltage and low on-voltage of a bipolar transistor, or excellent features, such as a lower speed than the MOSFET and a high-speed operation, and is an important semiconductor element which supports power electronics now.
However, the IGBT 101 illustrated in the main portion cross-sectional view of FIG. 6A has a reverse breakdown voltage junction (collector junction 103), unlike a MOSFET 301 illustrated in FIG. 6B. Therefore, in general, in the IGBT 101, a current cannot flow in a reverse direction (a bias direction in which an emitter E is a positive electrode and a collector C is a negative electrode). When the IGBT 101 is changed from an on state to a forward blocking state, a high surge voltage is likely to be generated in the reverse direction due to an inductance component in the circuit. When the surge voltage is applied to the IGBT 101, generally, there is a concern that the IGBT 101 which is not protected from a reverse breakdown voltage will be broken. However, when the IGBT is used in the inverter circuit, the IGBT is protected by a diode 401 (see FIG. 5) which is connected in inversely parallel in order to return an L load (dielectric load) current that is generated whenever the IGBT 101 is turned off. Reference numerals 102 and 302 indicate an n− drift layer.
There is an increasing demand for increasing the frequency of the inverter. The parallel connection of the IGBT 101 and the general free-wheeling diode 401 has a limitation in increasing a switching speed. Therefore, the IGBT 101 which can switch at a high speed and a fast diode are used in order to meet the demand. In the fast diode, the time required for reverse recovery when the diode is changed from a state in which a forward current flows to a reverse blocking state is shorter than that of a general diode. The use of the diode makes it possible to reduce reverse recovery loss.
FIG. 2A is a cross-sectional view illustrating a main portion of a super junction MOSFET according to the related art and is a carrier lifetime distribution diagram in which the vertical axis indicates a depth in correspondence with the depth direction of a substrate corresponding to FIG. 2A. In recent years, in order to further improve the speed of the switching element, the replacement of the IGBT 101 with a super junction MOSFET 201 illustrated in FIG. 2A has been examined. The super junction MOSFET 201 (see FIG. 2A) which has been examined as a replacement target has a super junction (SJ) structure having a drift layer 205 as a parallel pn layer in which a n-type region (hereinafter, referred to as an n-type drift region) 202a with high impurity concentration and a p-type region (hereinafter, referred to as a p-type partition region) 202b are alternately arranged in a direction parallel to a main surface of a substrate at a small interval (pitch). In addition, the drift layer includes a first n-type buffer layer 204 which is provided on the drain side of a parallel pn layer 202. When the carrier lifetime of the substrate is not controlled, the carrier lifetime is constant (not controlled) in the depth direction from the surface of the substrate, as illustrated in FIG. 2B. In the super junction MOSFET 201, even when impurity concentration is higher than general impurity concentration in order to match the n-type drift region 202a of the parallel pn layer 202 with the breakdown voltage, the pitch between the parallel pn layers 202 can be reduced to deplete all of the parallel pn layers 202 at a low voltage. Therefore, the super junction MOSFET 201 has the characteristics of a high-breakdown voltage and low on-resistance even though it is a unipolar type. In addition, the super junction MOSFET can perform high-speed switching resulting from a unipolar device and includes a reverse diode structure (reference numerals 203 and 202a in FIG. 2(a)). Therefore, it is not necessary to newly connect the parallel diode 401 of the inverter circuit illustrated in FIG. 5 and a reduction in the size of the device can be expected. In addition, the super junction MOSFET (SJ-MOSFET) 201 is used as a switching device and the built-in diode is used as a fast recovery diode to further increase the speed and to further reduce loss.
As a document related to the super junction MOSFET 201, a document which discloses the following structure has been published: an SJ structure including a parallel pn layer and an n-type buffer layer which is provided below the layer and in which impurity concentration is changed in two stages are provided in a drift layer 205 to reduce on-resistance and to form a built-in diode having a soft recovery waveform as reverse recovery characteristics (for example, see the following Patent Document 1). In addition, a semiconductor device has been known which has an SJ-MOS structure for shortening a reverse recovery time, without increasing a leakage current between a drain and a source (for example, see the following Patent Document 2). Furthermore, a structure has been proposed in which an SJ-MOSFET is connected to a Schottky barrier diode having an SJ structure to achieve a semiconductor device suitable for a soft switching type (for example, see the following Patent Document 3). A structure has been proposed in which a lifetime control region is provided in the entire Schottky barrier diode having an SJ structure to reduce a reverse current and to improve reverse recovery characteristics (for example, see the following Patent Document 4). A lifetime control method for obtaining reverse recovery characteristics with a soft recovery waveform (for example, see the following Patent Document 5). A method for controlling the lifetime of an excess minority carrier has been proposed (for example, see the following Patent Document 6). In addition, a semiconductor device has been proposed which can improve a breakdown voltage and turn-off characteristics, as compared to the elements according to the related art (for example, see the following Patent Document 7).